Poly(phenylene ether)s are commercially attractive materials because of their unique combination of physical, chemical, and electrical properties. Furthermore, the combination of poly(phenylene ether)s with other polymers or additives provides blends which result in improved overall properties including chemical resistance, high strength, and high flow. As new commercial application are explored for phenylene ether-containing materials, a wider range of intrinsic viscosity materials, particularly lower intrinsic viscosity materials, are desired.
The processes most generally used to produce poly(phenylene ether)s involve the self-condensation of at least one monovalent phenol in the presence of an oxygen containing gas and a catalyst comprising a metal amine complex to produce resins typically within the intrinsic viscosity range of 0.35 to 0.65 deciliter per gram as measured in chloroform at 25° C. These processes are typically carried out in the presence of an organic solvent and the reaction is usually terminated by removal of the catalyst from the reaction mixture. The catalyst metal, after being converted into a soluble metal complex with the aid of a chelating agent, is removed from the polymer solution with standard extraction techniques, such as liquid-liquid extraction.
Various techniques of isolating poly(phenylene ether)s from solution have been described. Some poly(phenylene ether)s have been isolated from solution by precipitation in an antisolvent, such as methanol. However, such precipitation methods often produce poor yields of poly(phenylene ether)s with low molecular weight and/or high relative concentrations of phenolic hydroxy groups. Some poly(phenylene ether)s have been isolated by a so-called devolatilizing extrusion process in which heat and reduced pressure in an extruder are used to drive off solvent. See, for example, U.S. Pat. Nos. 6,211,327 B1 and 6,307,010 to Braat et al. However, some poly(phenylene ether)s undergo thermal and/or oxidative degradation in these devolatilizing extrusion processes. Furthermore, evaporation of solvents is diffusion-limited and the desired melt thickness is not readily obtainable. It is therefore difficult to achieve low levels of residual solvent in the final product.
Other evaporative techniques, including wiped film evaporation (WFE) technology, have also been evaluated for poly(phenylene ether)s. WFE typically faces problems including undesired foaming, entrainment, and inconsistent solvent devolatilization. WFE techniques are also not generally suitable for isolating phenylene ether oligomers. In a wiped film evaporator, there is physical contact between the rotating blades of the evaporator and the surface of the evaporator. Thus, WFE is generally preferred for processing low viscosity solutions (e.g., solutions having a viscosity of less than 10,000 centipoise), as shown in FIG. 1, where shaded region B indicates the preferred melt viscosity ranges for various phenylene ether materials. The rotating blades of the WFE are continually wiping the material from the surface of the WFE, precluding the formation of a uniform film coating the interior of the WFE cylinder. Furthermore, the inherent risk associated with the presence or formation of solid particles during the poly(phenylene ether) isolation procedure requires that the edges of the WFE, rotor blades be cushioned with hinges. The presence of additional moving parts translates to higher wear on the equipment, which can be costly and lead to excessive down time. Additionally, agitator blades of a WFE require cushioning with carbon bushings, the presence of which can lead to feed retention. The degradation of retained material combined with wear of the carbon bushing can lead to formation of high levels of black specks which can contaminate the desired phenylene ether product.
There remains a need for an improved process for the isolation of a phenylene ether oligomer that overcomes the above-described limitations of known processes. Such a process would desirably provide a phenylene ether oligomer composition that meets various property specifications, specifically related to residual solvent levels, intrinsic viscosity, and black speck contamination. Additionally, a preferred process would further allow for tuning of the amount of amine content present in the phenylene ether oligomer composition.